1. Field of the Invention
This invention relates to the field of programmable microprocessor controlled electronic key telephone systems which feature digital data communication for control signalling between common control central processor elements and station instruments, and which thereby achieve a reduction in the associated station cable pairs required and increase the system feature capability.
2. Description of the Prior Art
Key telephone systems make it possible for a number of station positions to share the use of several incoming C.O. or PBX telephone lines. Similarly, key systems provide for the termination or appearance of more than one telephone line at the station location, thereby providing the station user with multiple line access to the outside. Intercommunication among stations of the key telephone system is provided by separate intercom paths and an auxiliary intercom switching matrix. Outside communications over these intercom paths generally is not possible. Buttons or keys at each telephone station provide manual selection and access to the associated telephone lines, and are used for control of the intercom paths, hold functions, paging, or other special service features furnished by the system. Normally, any incoming call over an outside C.O. line can be answered at any one of the telephone stations having an appearance of that particular line.
In contrast to the more sophisticated controls of private branch exchange systems, control of conventional key telephone systems is distributed throughout the system, and is usually manually initiated through the actuation of the corresponding line, intercom, or feature service button or keys.
Over the past three decades, key telephone systems employing conventional electro-mechanical technology have achieved widespread use, with nearly 13 million key station telephones currently in-place throughout the United States.
Over the years, the combination of the rudimentary form of distributed control ordinarily employed by conventional electro-mechanical key telephone systems, coupled with the desire to increase the number of outside lines, intercom paths, and special features appearing at each telephone station position, has resulted in enormous cabling requirements between the Key Service Unit (KSU) and the associated multi-line telephone instruments. This, in turn, has not only increased the material cost of conventional electro-mechanical key telephone systems, but it has also contributed to higher installation and maintenance labor costs required to sort out, connect, repair and otherwise maintain the complex array of premises wiring which is ordinarily associated with such systems.
With the advent of competition in the telecommunications terminal equipment industry over the last decade, new generations of electronic key telephone systems have been introduced. Generally, the new electronic systems have focused on efforts to reduce escalating cabling costs, and increase feature service capability. Unfortunately, efforts to achieve these objectives have not always been without offsetting costs of other types. For example, reduction in station cabling has been achieved only by combining greater use of centralized control coupled to sophisticated electronic stations by means of a data link. Many design solutions to the cable reduction problem have introduced time division multiplex switching (TDM) techniques at the common equipment for switching the station talk paths to the appropriate outside line or intercom path. While multiplexing the station speech paths to the telephone lines may help achieve a desired reduction in station pairs, it introduces an interruption in the direct metallic connection between the station and the outside line, and requires fairly high-powered central processor control and system programming. In addition, whether time division multiplex switching or space division switching is employed, electronic key system central processors usually require comparably sophisticated electronic multi-button station instruments in place of those of conventional design.
Not all electronic key systems have achieved cable reduction with the same degree of efficiency. While virtually all elctronic key systems have succeeded in reducing the cabling requirements below that formerly required with the electro-mechanical technology, not all electronic systems have achieved the same level of reduction. Many systems require a separate cable pair to furnish power to the additional station electronics. Other systems require a separate data pair for the transmission of data to the station and another data pair for the transmission of information from the station. Yet other electronic key systems require a dedicated cable pair terminated on station key buttons reserved for implementing special feature services.
The trend toward the use of data signalling between the common equipment central processor and the electronic station instruments has introduced environmental considerations which may affect the operation of these electronic key systems, or which may even restrict their use in many user applications. The transmission of high-speed data over the data links is vulnerable to the introduction of noise and other sources of possible data error, and requires introduction of highly sophisticated error detection and encoding schemes into the key system design previously familiar only to personnel in the data processing and computer fields. The installation, maintenance, repair and field assembly of these systems poses strange and unfamiliar problems to field personnel accustomed to conventional telephone technology.
It is therefore desirable in the design of electronic key systems to achieve the necessary cost-saving reductions in excessive cable material and labor costs, preserve a system identity with conventional telephone terminal equipment electro-mechanical technology, and yet offer the maximum feature flexibility and other economies afforded by the state-of-the-art integrated circuit and microprocessor electronic technologies.
Accordingly, it is one object of this invention to reduce the total number of station cable pairs ordinarily required between the key system KSU or common equipment, and the station position. More specifically, it is a further object of this invention to minimize the number of pairs required to no more than two pairs--one pair for both data and station power, and one pair for speech signals.
Another object of this invention is to improve the efficiency and reliability, and reduce the cost of data signal transmission between the KSU central processor and the station positions by means of bi-directional serial bit stream control over a digital data communication link.
Another object is to reduce the number of key system circuits and components required to provide special key system service features, by initiating such features using non-locking station-associated buttons as multi-function switches, and programming the central processor to respond to sequential operation of said switches.
Still another object of this invention is to reduce total key system costs by avoiding the need for expensive multi-button electronic telephone instruments ordinarily required for use with electronic key systems.